It is known that the mechanical properties of titanium can already be improved by means of alloying additions. By the addition of certain alloying elements the transformation temperature of titanium from the .alpha. into the .beta. phase can be raised or lowered, i.e., a distinction is made between alloying additions that stabilize either the .alpha. or the .beta. phase. For example, aluminum is among the .alpha.-stabilizing alloying elements and is dissolved as a substitutional mixed crystal, while vandium and molybdenum, among others, can be cited as prime examples of .beta.-stabilizing alloying elements. Zirconium and tin dissolve well in both phases.
The different phases present at room temperature after annealing are subdivided into .alpha.-titanium alloys, .beta.-titanium alloys and (.alpha.+.beta.)-titanium alloys. These alloys are described by, for example, A. D. McQuillan and M. K. McQuillan in "TITANIUM", London, Butterwords Scientific Publications, 1956.
The present invention relates especially to the (.alpha.+.beta.) titanium alloys. Typical examples of these alloys are the alloys listed in Table I below, for which the strength data at room temperature are also indicated.
TABLE I ______________________________________ Ultimate 0.2%-offset Elongation Reduc- tensile yield after tion strength strength fracture of area RM R.sub.P0.2% EL RA (.alpha. + .beta.) alloy [MPa] [MPa] [%] [%] ______________________________________ Ti4Al4Mo2Sn 0.5Si 1115 980 9 20 Ti6Al4V 900 830 10 20 Ti6Al6V2Sn Fe Cu 1035 965 10 15 Ti6Al4Zr2Mo2Sn 900 830 10 20 Ti7Al4Mo 965 900 10 15 ______________________________________
In recent years there has been no lack of attempts to improve the static and dynamic mechanical properties of these (.alpha.+.beta.) titanium alloys by subjecting them to special treatments, i.e., thermomechanical treatments, wherein the materials are first usually hot-worked, since their elongation before reduction of area is small. By means of solution annealing and stabilization, it is then possible to achieve better material properties such as, for example, increased thermal stability and improved creep behavior.
Numerous publications concerning improvements of the mechanical properties of titanium alloys have recently appeared in connection with the International Conference on Titanium of Sept. 10-14, 1984 in Munich in Volume 1 of the Proceedings. By way of example, reference is made here to the papers in that Volume 1 on page 179 ff., page 267 ff., page 327 ff. and page 339 ff. The mechanical properties of highly advanced PM titanium shaped parts are also reported by J. P. Herteman et al. in "Powder Metallurgy International" Vol. 17, No. 3, 1985, pages 116 to 118, wherein the authors have observed that the mechanical properties of a material processed by hot isostatic pressing can be improved by the use of purer oxide-free powder and the adjustment of a suitable structure to such an extent that this so-called HIP material, in its strength values and susceptibility to damage, can be favorably compared with forged materials or is even slightly superior to them. Nonetheless, however, that paper reveals that the values for the ultimate tensile strength (RM) and yield strength (0.2%-offset yield strength R.sub.P0.2%) still cannot be raised above 1100 MPa, while the elongation (breaking elongation EL) does not rise above 17% and the reduction of area (RA) reaches hardly more than 40%.